Electric salt bath furnace



May 23, 1944.

H. A. ROLNICK ELECTRIC SALT BATH FURNACE 2 Sheets-Sheet 1 Filed July 17,1943 4 INVENTOR. fiQ/PRYA- Ram/wan May 23, 1944.

H. A. ROLNICK ELECTRIC sum BATH rummcn Filed July 1'7, 1943.. 2Sheets-Sheet 2 uvmvrox A #04 zv/c/r ATTORNEY Patented May 23, 1944ELECTRIC SAL-T BATH FURNACE Harry A. Rolnick, Philadelphia, Pa.,assignor to Rolnick Testing & Manufacturing 00., Philadelphia, Pa.

Application July 17, 1943, Serial No. 495,132

4 Claims.

This invention relates to electrically heated salt bath furnaces whichare being increasingly used, particularly for the hardening of metalssuch as high speed steel and aluminum. The reason for the increasing useof these furnaces for this purpose resides in the fact that the moltensalt when heated by electrical current is found to form a very desirableheat transmitting medium. A major problem has arisen, however, in thepractical employment of such furnaces, which is the maintenance ofuniform temperature throughout the bath. This is a fundamental requisiteof a heating furnace which is to be practical and successful, as will beunderstood when it is stated that at a temperature of 920 F. a variationin excess of ma make the difference between success and failure in theheating of aluminum parts.

Heretofore the maintenance of such uniform temperature has not beenachieved by the method of heat introduction generally employed. Thismethod consisted of introducing current into the salt bath by means of apair of electrodes extending from the top of the furnace downwardly intothe bath. An example of the results achieved by this arrangement is asfollows: For hardening aluminum alloys I employed a salt bath furnace 8long, 5 deep, and 3' wide, with standard electrodes arranged in thestandard manner, that is, the electrodes extended from the top of thefurnace downwardly into the bath along one side of the furnace. I thendetermined by measurement that the temperature at the top of the bathwas 920 F., the temperature 2' below the top was 910 F., the temperature3 below the top was 870 F., while at the bottom of the bath the tem'perature was 750 F. Such a temperature differential from top to bottomof the salt bath is highly objectionable because the temperature atwhich the hardening of aluminum is carried out must range between 910 F.and 930 F.

It is therefore the principal object of my invention to provide anelectric salt bath furnace wherein substantial uniformity of temperatureis obtained throughout the bath. For this purpose I take maximumadvantage of the natural convection currents which are caused whenmolten salts with a lower density rise and are replaced by the coldersalt possessing a higher density. To accomplish the foregoing result Iprovide an arrangement of electrodes whereby substantially all of theheat in the bath is generated closely adjacent the bottom. By thisarrangement I obtain in the same furnace hereinbefore described, theremarkable temperature uniformity within 4 F. from top to bottom of thesalt bath as against the variation of F. with the stand-. ardarrangement of standard parts.

Further objects and advantages of this invention will become apparent inthe following de tailed description thereof:

In the accompanying drawings,

Fig. 1 is an isometric projection, with parts broken away, of anelectric furnace embodying one form of my invention.

Fig. 2 is a section taken substantially on the line 22 of Fig. l.

Fig. 3 is a view similar to Fig. l, but showing a modified form ofelectrode.

Referring to the drawings, there is shown an.

electric salt bath furnace comprising a chamber H), which in Fig. 1 hastwo of the walls broken away for the purpose of disclosing the interiorthereof. The chamber, which may be made entirely of ceramic,diatomaceous, or other earth materials which are heat resisting, isadapted to contain the melt consisting of a mixture of salts which fusesat the desired high temperature to form a molten mass or bath. The heatfor melting the salts and maintaining the bath at the desiredtemperature is provided by electrodes indicated generally at H and I2,and extending downwardly into the bath. One or more pairs of electrodesmay be employed. The electrodes are connected at their upper ends to asuitable source of alternating current. Heat is generated mainly alongthe adjacent edges of the electrodes, and the quantity of heat generateddepends upon the distance between these edges, the length of the edges,and the current density.

I-Ieretofore, as described in the introduction hereto, the electrodeswere placed with their adjacent edges relatively close together, andthey extended downwardly to a predetermined distance above the bottom ofthe bath. The net result was a temperature gradient which was higher atthe top and lower at the bottom, and in the instance cited hereinbefore,amounted to a 170 F. differential. As a result, the lighter salt bathmixture was formed at the top and the denser mixture at the bottom.

My invention operates on a principle exactly opposite to that describedabove, and as a result, I am enabled to obtain a salt bath mixture wherethe lighter elements constantly tend to form at the bottom, thus tendingto rise and be replaced by the denser mixture from the top. A maximumutilization of convection currents is thus effected, which results inremarkable uniformity of temperature. In actual figures, a bath whichdoes not vary more than :4" F. throughout its entire volume has'beenobtained in the same furnace which heretofore was subject to a 170 F.variation under the same operating conditions,

For carrying out the above novel principle of my invention, I causesubstantially all of the heat in the bah to be generated closelyadjacent, and substantially parallel, to the bottom of the bath. 1 canachieve this result in several ways. As shown in Fig. 1, I may form theelectrodes of the usually downwardly extending portions I4 and I5, butinstead of terminating the electrodes at the bottom of these portions, Iprovide the downwardly extending portions I4 and I5 with substantiallyhorizontal extensions I6 and I! adjacent and substantially parallel tothe bottom of the bath. The downwardly extending portions I4 and I5 arepositioned as far apart as possible so that'relatively little currentwill passbetween them, while the horizontal extensions I6 and I! are soformed that their adjacent edges I9 and 20 are relatively close to carrysubstantially all of the current. Under these conditions practically allof the heat will be generated closely adjacent the bottom. The lighterheated mass will rise and be replaced continuously by the cooler,heavier mass from thetop, thus producing maximum convection currents,insuring uniformity of temperature throughout the bath. Such uniformityis the highly desired goal in all salt bath furnaces.

In a modified form of my invention I may secure the. same result byinsulating the downwardly extending portions of the electrodes so thatcurrent can pass only between the adjacent edges of the horizontalportions of the electrodes. In this formof the invention it would not benecessary to space the downwardly extending portions I4 and I5 as farapart as shown in Fig. 1, if such spacing were not otherwise desired.

In the form of the invention where the downwardlyextending portions arenot insulated there will be a tendency for a relatively small amount ofcurrent to pass between these portions. To compensate for this tendency,I may form the horizontal portions of the electrodes as shown in Fig. 3,at I6, and IT, with the parts of the horizontal portions farthest awayfrom the downwardly-extending portions being positioned closest togetherand spreading apart as they approach the downwardly-extending portions.This will give a toeing-in effect so that more current will pass andmore heat will be increasingly generated adjacent the bottom the furtherthe distance from the downwardly-extending portions.

v In' accordance with the provisions of the patent statutes, I haveherein described the principle and operation of my invention, togetherwith the apparatus which I now consider to represent the best embodimentthereof, but I desire to have it understood that the apparatus shown isonly illustrative, and that the invention can be carried out by otherequivalent means. Also, while it ments in the combination and relationsdescribed, some of these may be altered and others omitted withoutinterfering with the more general results outlined, and the inventionextends to such use.

Having described my invention, what I claim and desire to secure byLetters Patent is:

1. An electric salt bath ,furnace comprising a plurality of electrodesfor passing current through the bath and adapted to be connected to asource of current supply, said electrodes having portions extendingdownwardly toward the bottom of the bath and having portions connectedto said first portions and extending in a single plane substantiallyparallel to the bottom of the bath, said last named portions beingpositioned with their adjacent edges relatively close togenerate largecurrents and produce vigorous thermal circulation.

2. An electric salt bath furnace comprising a plurality of electrodesfor passing current through the bath and adapted to be connected to asource of current supply, said electrodes having portions extendingdownwardly toward the bottom of the bath and having portions connectedto said first portions and extending in a single plane substantiallyparallel to the bottom of the bath and substantially the full width ofthe bath, said last named portions being positioned with their adjacentedges relatively close to generate large currents and produce vigorousthermal circulation.

3. An electric salt bath furnace comprising a plurality of electrodesfor passing current is designed to use the various features andelethrough the bath and adapted to be connected to a source of currentsupply, said electrodes having portions extending downwardly toward thebottom of the bath and having portions connected to said first portionsand extending in a single plane substantially parallel to the bottom ofthe bath, said last named portions being positioned with their adjacentedges relatively close to generate large currents and produce vigorousthermal circulation, the electrodes being so formed that the adjacentedges of the laterally- I extending portions are closer to each otherthan the adjacent edges of the downwardly-extending portions.

4. An electric salt bath furnace comprising a plurality of electrodesfor passing current through the bath and adapted to be connected to asource of current supply, said electrodes having portions extendingdownwardly toward the bottom of the bath and having portions connectedto said first portions and extending in a single plane substantiallyparallel to the bottom of the bath and substantially the full width ofthe bath, said last named portions being positioned with their adjacentedges relatively close to generate large currents and produce vigorousthermal circulation, said adjacent edges being positioned closertogether the, further their distance from the downwardly-extendingportions.

HARRY A. ROLNICK.

